[Server] Fix memory growing with every tools/call

CHANGELOG.md

@@ -14,6 +14,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * [BC Break] Reject a `Tool` input schema whose `properties` is not an object or whose `required` is neither a list nor `null`, instead of silently replacing the member. Reject a `completion/complete` whose `argument` is missing `name` or `value`, instead of completing against an empty prefix.
 * Add `HttpTransport::getSessionId()` to read the server-minted `Mcp-Session-Id`: a request-scoped caller can persist it and pass it back through the constructor's `$headers` on a later transport. Always `null` on `2026-07-28`, which removed protocol-level sessions.
 * Fix `HttpTransport` omitting the `MCP-Protocol-Version` header on requests after the `initialize` handshake, including the closing `DELETE`.
+* Fix memory growing with every `tools/call` in long-lived processes like FrankenPHP workers or stdio servers.
 * Fix OIDC discovery rejecting issuers with a trailing slash (e.g. Authentik, Auth0).
 * Fix `Client::connect()` rejecting an empty `serverInfo.version`, as the Python SDK sends, and leaving the transport open when initialization fails on a malformed result: the transport is now closed and the failure wrapped in a `ConnectionException`.
 * Fix stateless SSE streams holding back frames until close when PHP output buffering is enabled.

src/Capability/Discovery/SchemaValidator.php

@@ -11,7 +11,6 @@
 
 namespace Mcp\Capability\Discovery;
 
-use Mcp\Exception\InvalidArgumentException;
 use Opis\JsonSchema\Errors\ValidationError;
 use Opis\JsonSchema\Validator;
 use Psr\Log\LoggerInterface;
@@ -30,6 +29,11 @@
  */
 class SchemaValidator
 {
+    /**
+     * @var array<string, mixed>
+     */
+    private array $decodedSchemas = [];
+
     public function __construct(
         private ?Validator $jsonSchemaValidator = null,
         private LoggerInterface $logger = new NullLogger(),
@@ -48,16 +52,9 @@ public function validateAgainstJsonSchema(mixed $data, array|object $schema): ar
     {
         try {
             // --- Schema Preparation ---
-            if (\is_array($schema)) {
-                $schemaJson = json_encode($schema, \JSON_THROW_ON_ERROR | \JSON_UNESCAPED_SLASHES);
-                $schemaObject = json_decode($schemaJson, false, 512, \JSON_THROW_ON_ERROR);
-            } elseif (\is_object($schema)) {
-                // This might be overly cautious but safer against varied inputs.
-                $schemaJson = json_encode($schema, \JSON_THROW_ON_ERROR | \JSON_UNESCAPED_SLASHES);
-                $schemaObject = json_decode($schemaJson, false, 512, \JSON_THROW_ON_ERROR);
-            } else {
-                throw new InvalidArgumentException('Schema must be an array or object.');
-            }
+            // Opis caches every schema object it sees for good, so the same schema has to map to the same object.
+            $schemaJson = json_encode($schema, \JSON_THROW_ON_ERROR | \JSON_UNESCAPED_SLASHES);
+            $schemaObject = $this->decodedSchemas[$schemaJson] ??= json_decode($schemaJson, false, 512, \JSON_THROW_ON_ERROR);
 
             // --- Data Preparation ---
             // Opis Validator generally prefers objects for object validation
@@ -66,10 +63,6 @@ public function validateAgainstJsonSchema(mixed $data, array|object $schema): ar
             $this->logger->error('MCP SDK: Invalid schema structure provided for validation (JSON conversion failed).', ['exception' => $e]);
 
             return [['pointer' => '', 'keyword' => 'internal', 'message' => 'Invalid schema definition provided (JSON error).']];
-        } catch (InvalidArgumentException $e) {
-            $this->logger->error('MCP SDK: Invalid schema structure provided for validation.', ['exception' => $e]);
-
-            return [['pointer' => '', 'keyword' => 'internal', 'message' => $e->getMessage()]];
         } catch (\Throwable $e) {
             $this->logger->error('MCP SDK: Error preparing data/schema for validation.', ['exception' => $e]);
 

tests/Unit/Capability/Discovery/SchemaValidatorTest.php

@@ -13,6 +13,8 @@
 
 use Mcp\Capability\Attribute\Schema;
 use Mcp\Capability\Discovery\SchemaValidator;
+use Opis\JsonSchema\SchemaLoader;
+use Opis\JsonSchema\Validator;
 use PHPUnit\Framework\TestCase;
 
 class SchemaValidatorTest extends TestCase
@@ -24,6 +26,28 @@ protected function setUp(): void
         $this->validator = new SchemaValidator();
     }
 
+    public function testRepeatedValidationDoesNotGrowSchemaLoaderCache(): void
+    {
+        $loader = new class extends SchemaLoader {
+            public function cacheSize(): int
+            {
+                return \count($this->dataCache) + \count($this->uriCache);
+            }
+        };
+        $validator = new SchemaValidator(new Validator($loader));
+
+        $validator->validateAgainstJsonSchema($this->getValidData(), $this->getSimpleSchema());
+        $validator->validateAgainstJsonSchema($this->getValidData(), (object) $this->getSimpleSchema());
+        $size = $loader->cacheSize();
+
+        for ($i = 0; $i < 10; ++$i) {
+            $validator->validateAgainstJsonSchema($this->getValidData(), $this->getSimpleSchema());
+            $validator->validateAgainstJsonSchema($this->getValidData(), (object) $this->getSimpleSchema());
+        }
+
+        $this->assertSame($size, $loader->cacheSize());
+    }
+
     // --- Basic Validation Tests ---
 
     public function testValidDataPassesValidation(): void